Deep‐Sea Hydrothermal Vent Mussels: Nutritional State and Distribution at the Galapagos Rift

Rich faunal communities have been discovered around active deep—sea hydrothermal vent sites. The mussel Bathymodiolus thermophilus (Bivalvia, Mytilidae) is one dominant member of this hard—substrate community along the Galapagos Rift, east of the Galapagos Islands in the equatorial eastern Pacific. These mussels are most dense around the vent effluent and decrease in number with increased distance away from the discharge site. At the Mussel Bed vent, at 2490 m depth, the nutritional state of mussels (oxygen consumption and chemical composition) from a densely populated site °1 m from a warm—water fissure was compared to that of mussels from a sparsely populated peripheral site to examine the importance of food supply on mussel distribution. The submersible Alvin was used to collect bottom—water samples for chemical analysis, to manipulate in situ respirometers, and to collect specimens for chemical composition analysis. The quantity of particulate organic carbon immediately above the bottom (<10 cm) was twice as high (207.0 mg/L) at the dense site at the peripheral site (106.4 @m/L). Other parameters measured showed no significant variation (P > .05) between sites, including water temperature, current speed, and concentrations of dissolved oxygen, nitrate, and dissolved organic carbon. Concentrations of ammonium, nitrite, and hydrogen sulfide were below the detection limits of the assay used. Oxygen comsumption rates (R) as a function of mussel dry mass (M) were higher at the dense site (R = 0.23M⁰ ³ ⁸) than at the peripheral site (R = 0.098M⁰ . ⁴8). Dense—site mussels had robust soft—tissue structure whereas peripheral mussels had an emaciated appearance. Water content was lower but condition index values and glycogen content were higher in the dense mussels. Less distinct differences were noted in total lipid and protein content between populations. Mussels examined 312 d after reciprocal site—to—site transplanting conformed in water and glycogen content to their adopted populations. I conclude that the nutritional state of the dense—site mussels is considerably higher than that of the peripheral mussels and is probably related to increased nutritive supply at the dense site. Possible causative factors are differences in the nutritive quality of organic matter and influences of symbiotic chemoautotrophs in the gills of these mussels.